Method and apparatus for measuring the dew point of a gas

Abstract

A method for measuring the dew point according to patent application P 36 33 015 is expanded so that an automatic zero point correction can be carried out. For this purpose, the heating power on the heating device is raised to a point, at which the dew evaporates and the sensor-specific property, for example the capacity, is measured and supplied to the evaluating unit as a calibration signal.

What is claimed is: 
     
       1. A method for measuring the dew point using a unit including a sensor which outputs a signal corresponding to a thickness of a dew layer on a surface exposed to the atmosphere to be examined, and a tempering device connected to the sensor including a heater and a cooler comprising the steps of: cooling and heating the sensor to bring the sensor to a temperature which leads to the formation of a dew layer on the surface of the sensor; detecting the dew layer by measuring the signal of the sensor; measuring the temperature of the sensor as the dew point temperature and calibrating the unit by heating the sensor surface to the point of completely drying the layer of dew, measuring the sensor output and comparing successive measurements of the sensor output to detect a zero rate of change of the sensor output corresponding to a zero point calibration value. 
     
     
       2. A method according to claim 1, further comprising the steps of: detecting changes in the thickness of the dew layer by detecting changes in the sensor signal; restoring the thickness of the dew layer by heating and/or cooling the sensor; and, measuring the temperature of the sensor, upon restoring the thickness of the dew layer, as the new dew point temperature. 
     
     
       3. A method of measuring the dew point using a unit including a sensor outputting a signal representative of a value corresponding to the thickness of a dew layer on a surface of the sensor exposed to the atmosphere to be examined, and a tempering device connected to the sensor including an electric heating unit and a cooling unit in constant contact with said sensor, comprising the steps of: cooling the sensor to a base temperature below the dew point; heating the sensor until the temperature difference between the base temperature and the dew point temperature is reached resulting in a dew layer of constant thickness being generated on the surface of the sensor element; detecting the dew layer by measuring the signal of the sensor; detecting changes in the thickness of the dew layer on the sensor, due to temperature change or change in the vapor contents of the atmosphere; restoring the dew layer on the sensor to its original constant thickness; measuring the temperature of the sensor having a dew layer of constant thickness as the dew point temperature; and, calibrating the unit by heating the sensor surface to the point of completely drying the layer of dew, measuring the sensor output and comparing successive measurements of the sensor output to detect a zero rate of change corresponding to a zero point calibration value. 
     
     
       4. A measurement device for measuring the dew point of a gas, comprising: a substrate, a comb-like formation of a conductor positioned on said substrate forming a sensor, said sensor having a sensor connection on said substrate; a heating device wound around said sensor having heating connections; a cooling device in thermal contact with said substrate; a temperature sensor positioned adjacent said sensor; and regulating means connected to said heating device, connected to said cooling device, connected to said sensor and connected to said temperature sensor for controlling cooling and heating of said sensor by way of said heating device and said cooling device, for receiving an output signal from said sensor representing the thickness of a dew layer on a surface of said sensor and for receiving an output signal from said temperature sensor and calibration means for establishing a zero point calibration value by controlling said heating device to heat the sensor surface to the point of completely drying the layer of dew, and for receiving the sensor output and comparing said successive sensor outputs to detect a zero rate of change in the sensor output corresponding to a zero point calibration value.